1. Field of the Invention
The present invention relates to a radio communication device which use at least one communication link for one-channel signal transmission.
2. Description of the Related Art
Bluetooth radio system (hereinafter simply called “Bluetooth”) is known as a radio communication device which uses at least one radio communication link for one-channel signal transmission.
This system is a radio communication system which interconnects devices placed quite close to each other, for example, a portable telephone and a notebook type personal computer, a stereo system and a headphone, and the like through a radio communication link using very weak radiowaves in a 2.4 GHz band, which does not require a license, to transmit data and audio.
For transmitting an audio signal in Bluetooth, the audio signal is converted to a digital signal by an audio codec (coder/decoder), generally employed in PCM communications for transmission. For quantization and encoding involved in the digital conversion, radio communication systems generally employ a logarithmic quantization scheme which implements 8-bit logarithmic quantization of an audio signal at a sampling frequency of 8 kHz, or a CVSD (Continuous Variable Slope Delta modulation) scheme which implements 1-bit quantization of an audio signal at a sampling frequency of 64 kHz. Therefore, the data transmission rate is 64 kb/s whether an audio signal is transmitted in accordance with any of the two scheme.
In the Bluetooth-based data transmission, devices which make communication to each other form a network called a “piconet” for mutual communications. The piconet includes a device called a master which takes the leadership within the piconet. The remaining devices within the piconet other than the master are called slaves which transmit data with the master under the leadership of the master.
The data transmission between the master and a slave is based on a time slot (625 μS) on the time axis which is mutually used by them in time division. Specifically, the data transmission between the master and slave employs a TDD (Time Division Duplexing) scheme which alternates transmission and reception for a half-duplex communication.
There are two types of communication links which are set between a master and slaves for data transmission: an SCO (Synchronous Connection Oriented) link (communication link for synchronous connection) and an ACL (Asynchronous Connection Less) link (communication link for asynchronous connection). The SOC link refers to a link formation which fixedly uses predetermined time slots for data transmission. The ACL link in turn refers to a link formation which utilizes an unused slot, each time data is transmitted, to specify the address of a slave which should receive the data. Generally, the SCO link is often utilized for audio data transmission because the real time capability is required.
There are three types of packet modes HV1, HV2, HV3, as data packets for use in the SCO link, according to the proportion of transmission data to an error correcting code added to the data. Out of these modes, an HV3-mode packet consists only of 240 bits of data without any error correcting code and therefore provides the highest transmission efficiency. For this reason, the HV3 mode is often used for the SCO links intended for audio transmission.
The two audio encoding schemes in Bluetooth, i.e., logarithmic quantization and CVSD-based quantization both assume that only audio signals are transmitted in a frequency band from 0 to 4 kHz. Specifically, when the logarithmic quantization is used, an audio signal is subjected to 8-bit logarithmic quantization after it is A/D converted at sampling frequency of 8 kHz, so that the signal frequency band is necessarily limited to 4 kHz. On the other hand, when the CVSD scheme is used, an audio signal is sampled at sampling frequency of 64 kHz, however, the band must be limited to 4 kHz before an audio signal is input to a CVSD encoder circuit. Since the CVSD scheme relies on a waveform gradient adaptive delta modulation, the limitation of the band is required for removing unnecessary high frequency components for suppressing the occurrence of so-called gradient distortion.
However, it is believed that an upper limit of human's audible frequencies is generally near 20 kHz, so that an audio band from 0 to 4 kHz cannot provide a sufficient audio quality at all times. Actually, the audio transmission using the SCO link of Bluetooth lacks clearness and natural impression and often loses inherent tonal characteristcs of speakers. Therefore, the transmission of audio signal over a wider frequency band is highly demanded in some communication applications.